Using Rotary Encoders
The electromechanical nature of the rotary encoder requires that it interface with mechanical devices or the user. When encoders are used as control interfaces on electronic devices, they use a solid shaft and are usually mounted on a control panel using a panel-mount bushing with associated hardware.
Designers can choose options such as detents to cause a mechanical “click” as the encoder is rotated, giving users tactile feedback that the encoder shaft is moving. They can also choose a momentary contact switch that is activated by pressing the encoder shaft. Encoders intended to be mounted on rotating machines like motors or servos have hollow or blind shafts (Figure 4).
A hollow shaft encoder mounts over the shaft of a motor or similar mechanical device. This ensures it is mounted concentrically with the device being monitored and eliminates the chance of asymmetric or angular misalignment. Blind shafts are hollow shafts with limited depth used to mount the encoder on the end of a motor shaft.
Encoder Selection and Application
The selection of a rotary encoder depends on the application requirements and the environment, as well as cost constraints. The 291V1022F832AB from CTS Electronic Components is an optical incremental encoder with 8 PPR angular resolution that runs off a 5-Volt supply (Figure 5). The 291 Series supports PPR resolutions ranging from 4 to 64 PPR, with optional selection of shaft types and lengths, detents, and an integral switch. The encoder has a rated rotational life of up to 3 million cycles. The optical encoders are ideal for instrument control applications including medical and laboratory equipment, communication, industrial, HVAC, transportation, security, audio, and home entertainment equipment.
Bourns Inc.'s EMS22Q51-D28-LT4 is a 32-PPR to 256-PPR incremental magnetic encoder that operates from either a 5-or a 3.3-Volt supply. Like the previous encoders, it has a variety of available shaft and bushing configurations but has a rated rotation life of 50 million cycles. These encoders are ideal for use in harsh industrial environments in the presence of extreme temperature, moisture, and particle contamination. Also, like many encoders, connecting and using the device is straightforward (Figure 6).
The EMS22Q series has six pins — one each for supply and ground, one active low chip select that goes to the microcontroller or microprocessor, one index pin, and two data pins (A and B). The resulting quadrature output is shown in Figure 7.
Designers of devices for low-cost applications can make good use of a mechanical encoder such as TT Electronics’ EN11-HSM1AF15 20-PPR encoder. This encoder is part of the EN11 series that offers angular resolution of 15 or 20 PPR, a variety of shaft and bushing lengths, a choice of optional switches, and a selection of detent configurations. This encoder operates with a 5-Volt supply, is roughly one-tenth the price of optical encoders, and has a rotational life of 30,000 cycles.
Rotary encoders fill a need for quickly and efficiently sensing and digitizing angular rotation of a front panel control, robotic arm, or a rotating motor shaft. Incremental or absolute encoders provide a necessary interface to microprocessors or microcontrollers to allow sensing and control of electromechanical systems components.
This article was contributed by Digi-Key Electronics , Thief River Falls, MN.